Field of the Invention
The present invention relates to a transmission including a planetary gear mechanism for transmitting a driving force from a drive source.
Description of the Related Art
Conventionally, a transmission having a planetary gear mechanism having a sun gear, a pinion gear and a ring gear is available. For example, see Japanese Published Patent Application No. 2015-175463 (Patent Document 1). At the center of the planetary gear mechanism, the sun gear as an outer-toothed gear wheel is arranged. The pinion gears as a plurality of small outer-toothed gear wheels mesh with the outer periphery of the sun gear at an equal distance. Further, the outer periphery of the pinion gear meshes with the ring gear as an inner-toothed gear wheel. A rotating shaft of the pinion gear is mounted to a frame referred to as a “carrier”. Due to this configuration, the planetary gear mechanism can change driving directions and gear ratios by inputting and outputting using the rotating shafts of the three gears, namely, the sun gear, the pinion gear and the ring gear and by fixing the same.
Here, in the transmission having such a planetary gear mechanism as disclosed in Patent Document 1, an inter-shaft distance between a shift shaft and a counter shaft (output shaft) needs to be short for the purpose of weight- and space-saving. A shortened inter-shaft distance enables minimization of the size of each gear, thereby achieving weight- and space-saving.
However, the shortened inter-shaft distance between the shift shaft and the counter shaft necessarily reduces thickness of the case between bearings supporting the shafts, thereby requiring larger strength of the case. As a large load occurs particularly to the counter shaft due to meshing of the gears and then the case supporting the counter shaft receives this load, the case needs to have particularly large strength.